Inorganic pigments/colorants are currently widely used in many industries, especially in those of paints, inks, plastics, rubbers, ceramics, enamels, and glasses. These pigments may impart colouristic properties, and protect the coating from the effects of visible as well as ultraviolet and infrared light. For such applications, their properties like chemical and thermal stability, dispersibility, chromaticity, tint strength and covering or masking power are particularly important criteria to be taken into account in the selection of a suitable pigment.
Yellow is particularly important colour in the ceramic pigment field and the consumption of the yellow exceeds that of any other coloured pigments. There are three important yellow pigment families: tin vanadia yellows (References may be made to DCMA 11-22-4), praseodymium zircon (DCMA 14-43-4), zircon vanadia yellow (References may be made to DCMA 1-01-4). Other yellow ceramic pigments commonly used such as Pb2Sb2O7, PbCrO4, CdS are now being expelled from the market because of their toxicity (References may be made to J. A. Badenes, M. Llusar, M. A. Tena, J. Calbo, G. Monros, J. Eur. Ceram. Soc. 2002, 22, 1981-1990).
There exist a large number of inorganic pigments formulations in the state of art, in which transition metals have been employed as chromophores. In contrast, rare earths are quite sparingly used in the inorganic pigments. A notable exception is praseodymium in praseodymium zircon yellow (Dry colour Manufactures Association).
The use of praseodymium doped zirconium silicate crystals as a pigment for use in ceramic glazes was disclosed by C. A. Seabright in U.S. Pat. No. 2,992,123, July 1961. Since that time, there have been numerous patents issued for praseodymium doped zircons for ceramic applications and now it is manufactured in world wide. Stable pigment particles comprising praseodymium doped zirconium silicate particles, at least about 50 percent by volume of which range, from 0.2 to 2 μm size has been disclosed in a U.S. Pat. No. 5,316,570, May 31, 1994, which can be used for plastics and paints.
Reference may be made to U.S. Pat. No. 5,275,649 wherein, they claim a process for the preparation of environment-friendly inorganic yellow pigment based on praseodymium zircons (ZrSiO4: Pr), which can be applied to ceramics because of its thermal stability. However, in the process, mineralizer sodium chloride or sodium fluoride has been employed.
Reference may be made to U.S. Pat. No. 5,560,772 wherein, they claim a non-toxic yellow/orange pigment compositions well suited for the coloration of wide variety of substrates, for example paints, varnishes, ceramics, etc. comprise a major amount of zirconium oxide and an additive amount of cerium, praseodymium and or terbium values in the form of oxides. However, this pigment preparation requires high temperature for calcinations (1700° C.).
Reference may be made to U.S. Pat. No. 5,693,102 wherein, they claim oxonitrides with a perovskite of the general formula LnTaON2, where Ln is a rare-earth element, and exhibit yellow-orange to reddish brown in color with an enhanced brightness. These compounds are produced in the presence of mineralizing agent from the series of alkali metal or alkaline earth halides, by annealing a powder mixture consisting of a Ta (V) compound and a Ln compound in a reducing atmosphere of ammonia. However, drawback in the preparation of these perovskites, it is necessary to heat the starting material in the flow of toxic and inflammable ammonia gas for longer periods (20-60 hrs).
Reference may be made to U.S. Pat. No. 6,419,735, wherein they claim a process for the preparation of yellow samarium sesquisulphide pigment. The process consists of reacting samarium, trivalent rare earth metal, and alkali metal or alkaline earth metal compounds with a gaseous mixture of hydrogen sulphide and carbon disulphide. The compositions of the invention exhibit a strong yellow colour. However one of the shortcomings of this process is that it utilizes toxic gases for the synthesis of the pigment.
Reference may be made to Patent No: WO9800367 wherein, they claim yellow inorganic pigments consisting of double molybdates of cerium and of an alkali metal, of general formula, CeM(MoO4)2, in which M denotes an alkali metal, preferably sodium. The pigments disclosed in the process were synthesized by precipitation and calcination methods under nitrogen atmosphere. The crystallographic structure of the double molybdates of cerium pigment has been characterized as deformed Scheelite type and monoclinic type.
Reference may be made to Japanese Patent No. JP3954837 wherein, they claim a process for a yellow cerium pigment having a general formula: ACexLn1−xMo2O8 wherein x ranges from 0 to 1; A in the composition is at least one selected from the group consisting of Li, Na, K, Rb and Cs and Ln is at least one selected from the group consisting of Y, La, Gd and Lu.
Reference may be made to U.S. Pat. No. 2,479,330wherein, they claim a process for the synthesis of a series of yellow inorganic pigments, comprising oxides of alkaline earth, praseodymium, and transition metals of the general formula (i) Pr2MoTmxO6+δ(Tm=Ti or Zr and x=0 or 1) and (ii) APr2MoO7 [A=Mg, Ca, Sr or Ba], well suited for colouring applications of a wide variety of substrates for example paints, varnishes, plastics, ceramics etc.
Although some alternative yellow pigments based on cerium oxide and other transition metal oxides have been reported, but their chromatic properties are not satisfactory for industrial use (References may be made to T. Masui, H. Tategaki, N. Imanaka, J. Mater. Sci. 2004, 39, 4909-4911; Imanaka et al. Chem. Lett. 2005, 34, 1322-1323; T. Masui, S. Furukawa, and N. Imanaka, Chem. Lett. 2006, 35, 1032-1033; S. Furukawa, T. Masui, N. Imanaka, Journal of Alloys and Compounds 2008, 45, 640-643; P. Prabhakar Rao and M. L. P. Reddy, Dyes and Pigments, 2004, 63, 169-174; Giable George, L. Sandhya Kumari, V. S. Vishnu, S. Ananthakumar and M. L. P. Reddy, J. Solid State Chem. 2008, 181, 487-492).
Unfortunately, the inorganic pigments which are suitable for the above applications and which are in use today on an industrial scale, generally comprise of metals like cadmium, lead, chromium, cobalt etc, the use of which is becoming strictly controlled and are even prohibited, by legislation in many countries, due to their high toxicity. Thus, serious need arises to design environment friendly inorganic yellow pigments devoid of the above cited disadvantages and drawbacks.
Thus the novelty of the present invention is to provide a non-toxic yellow inorganic pigment based on mixed oxides of samarium and molybdenum compounds.